1. Field of the Invention
The present invention relates to a fuel injection control apparatus for correcting an amount of fuel injected by a fuel injection valve, and a fuel injection system including the fuel injection control apparatus.
2. Description of Related Art
There is known a fuel injection control apparatus configured to detect an actual fuel injection amount of a fuel injection valve on the basis of variation of a running state of an engine such as variation of an engine speed due to fuel injection, and correct a fuel injection amount depending on the difference between a commanded fuel injection amount directed to the fuel injection valve and the actual fuel injection amount. For example, refer to Japanese Patent application Laid-open No. 2005-36788.
Particularly, in the case of a diesel engine, it is necessary to perform a fuel injection amount learning in order to enable precisely correcting a fuel injection amount at the time of carrying out a small amount fuel injection (may be referred to as “pilot injection” hereinafter) to reduce NOx and combustion noise.
Since a fuel injection amount varies depending on the injection pressure, it is preferable to perform the fuel injection amount learning for each of different target injection pressures. Some of the conventional fuel injection amount control apparatuses are configured to perform the fuel injection amount learning by injecting fuel at a pressure deviated from a target injection pressure. In this case, since it is necessary to modify a learned injection amount correction value depending on the difference between the target injection pressure and the actual injection pressure, the correction accuracy of fuel injection amount is lowered.
In addition, since when predetermined conditions to allow the fuel injection amount learning to be performed are satisfied, the injection pressure is low in many cases, there is a problem that the range of the injection pressures at each of which the fuel injection amount learning is performed is biased toward the low-pressure side. It might occur that the injection pressure in the normal injection mode (may be referred to as “normal injection pressure”) is set to the target injection pressure at the time of performing the fuel injection amount learning.
However, in this case, if the target injection pressure is higher than the normal injection pressure, there may be a problem that the injection pressure is not reduced to a sufficient degree when the normal injection mode is resumed. If the normal injection mode is resumed in a state where the injection pressure is not reduced to a sufficient degree, there occurs a large variation in the engine running state such as combustion noise and vibration of the engine. This may give concern to the driver of the vehicle.
Moreover, if the target injection pressure is too high, since the combustion noise and vibration of the engine are large, the driver may feel concern during the fuel injection amount learning.